The invention relates to an information system comprising a record carrier on which information has been recorded in the form of variations of a first physical parameter and a playback apparatus provided with means for scanning the record carrier by means of a transducer which is responsive to said variations of said first physical parameter, means for recovering the information from a detection signal received from said transducer.
The invention further relates to a record carrier and a play-back apparatus for use in the system.
A system of the type mentioned above is inter alia known as the Compact-Disc system. Normal Compact Discs play on all compatible playback apparatuses. Nowadays recording apparatus are available for copying the information present on a read-only Compact Disc on a recordable disc which can be played on the compatible play-back apparatus.
However some applications , for example audio-visual games, require a so-called "closed system" in which the Compact Disc with the software representing the audio-visual game can only be played on special players and which cannot be copied easily by means of the available recording apparatuses.
In the view of cost price aspects it is desired that know-how of the already existing information systems can be used as much as possible. Therefore it is desired to amend existing information systems as less as possible in order to realize a low-cost "closed information system".
However it should be made very difficult for third parties to copy such special discs by means of existing copying machines. The prior art protection schemes do not meet this requirement, e.g.
rate scrambling/encryption can by copied from disc to disc with a bit copying machine;
special logical errors (to copy protect) in a main and/or subcode channel (of e.g. a Compact Disc) can also be copied with a bit copying machine;
schemes relying on the relation main/subcode channel can also be copied with a bit copying machine.